Metal halide arc discharge lamps are widely used for general illumination. These lamps include an arc tube mounted within a light-transmissive lamp envelope. The lamp envelope may be evacuated or backfilled with nitrogen. The arc tube has electrodes mounted at opposite ends and contains a fill material including a starting gas, mercury and one or more metal halides. A starting electrode is typically mounted at one end of the arc tube adjacent to one of the main electrodes to assist in starting. When the lamp is first turned on, a voltage is applied between the starting electrode and the adjacent main electrode. A discharge between the starting electrode and the adjacent main electrode heats the arc tube sufficiently to form a discharge between the main electrodes. After a discharge is formed between the main electrodes, the starting electrode is shorted to the adjacent main electrode by a thermal switch.
A thermal switch for metal halide lamps is disclosed in U.S. Pat. No. 3,965,387, issued Jun. 22, 1976 to Stuart et al. The disclosed thermal switch includes a bimetal element and a conductive spring element attached to the bimetal element. At temperatures above the closure temperature of the thermal switch, the spring element is deformed, thereby relieving some of the stress which would otherwise be applied to the bimetal element.
A thermal switch for an arc discharge lamp is disclosed in U.S. Pat. No. 4,659,965, issued Apr. 21, 1987 to Bonazoli. The disclosed switch includes a bimetal strip attached to one electrode lead and a spring-like member attached to another electrode lead. The bimetal strip has a notch to receive the spring-like member when the closure temperature is reached.
A thermal switch wherein a bimetal element moves with a sliding action along a resilient conductor as the temperature increases above the closure temperature so as to minimize stress and provide a self-cleaning action is disclosed in pending application Ser. No. 07/634,464, filed Dec. 27, 1990.
While prior art thermal switches provide generally satisfactory operation, certain practical problems have arisen in the use of thermal switches in compact, low wattage arc discharge lamps. In prior art lamps, the bimetal element is typically mounted laterally with respect to the longitudinal axis of the lamp envelope. In compact arc discharge lamps with a lamp envelope having a small diameter neck, the bimetal element interferes with installation of the arc tube assembly in the lamp envelope. Furthermore, the bimetal element may be damaged by the relatively high temperatures in compact arc discharge lamps.
One solution to this problem is disclosed in U.S. Pat. No. 5,015,916, issued May 14, 1991 to Mazza et al. The bimetal element is bent to avoid interference with the lamp envelope without shortening of the bimetal element However, the disclosed configuration may create undesired sets in the bimetal element and weaken the resilient conductor, and adds angles that must be controlled in production. In addition, it is believed that the thermal switch is subjected to excessive temperatures during operation in compact arc discharge lamps. When the thermal switch is repeatedly subjected to excessive temperatures, the closure temperature may be changed, or the thermal switch may fail to operate entirely.
It is a general object of the present invention to provide improved arc discharge lamps.
It is another object of the present invention to provide arc discharge lamps with improved mounting arrangements for thermal switches.
It is a further object of the present invention to provide arc discharge lamps wherein a thermal switch is mounted in a relatively cool location within the lamp envelope.
It is yet another object of the present invention to provide arc discharge lamps wherein a thermal switch is mounted to facilitate lamp assembly.
It is a further object of the present invention to provide arc discharge lamps wherein a thermal switch is mounted to avoid extreme operating temperatures.
It is still another object of the present invention to provide arc discharge lamps having a thermal switch mounting arrangement that is low in cost and easy to manufacture.